Chemistry Reference
In-Depth Information
conditions and spill age. For these reasons, identical spills can exhibit
different damping ratios on quasi-simultaneous Almaz-1 and ERS-1
SAR images.
2. Using high resolution Almaz-1 SAR images, for the first time an effect
of an intensification of short waves at the upwind side, i.e., the wind-
ward edge of an oil spill, was found and a possible explanation is given.
The overshoot effect can explain the intensification of short wave com-
ponents at the windward edge. The increase in radar backscatter at the
windward edge of the oil spill attained values between 1 and 2 dB. The
effect found on the Almaz-1 SAR images requires a theoretical explana-
tion as well as special additional experiments to understand these phe-
nomena in detail. Another Almaz-1 SAR image displays the short waves
intensification effect also within the spill area with increasing of the
NRCS up to 1-1.5 dB against the background. On the one hand, the ef-
fect was stipulated by the change of the wave development and interac-
tion regime (August 22
nd
; and, perhaps, August 27
th
). On the other hand,
the amplification within the spill area may be explained by film disinte-
gration as a result of ageing under the influence of environmental fac-
tors (August 27
th
).
To sum it up, the main conclusion is that the usual description of
crude oil spills in SAR images in terms of damping ratios is not well
enough understood. The radar signal damping in spills depends primarily
on the wind speed and age, and effects leading to the transformation of oil
properties, when a spill floats on the sea surface. Most of these factors are
not usually known when spills are observed from space, and the effects
mentioned are not sufficiently studied. All this imposes additional restric-
tions on the use of SAR for detection and classification of crude oil spills.
The experimental facts also show that spaceborne imaging radars are not
always able to detect crude oil spills in the ocean under low and high wind
conditions.
Achnowledgements.
The authors thank Dr. S. Ermakov for fruitful dis-
cussions of the paper, Mr. Terje Wahl of the Norwegian Defense Research
Establishment (NDRE) and Mr. Knut Iden of the Norwegian Meteorologi-
cal Institute (DNMI) for providing experimental data on weather and sea
state. This work was supported by Russian Foundation for Basic Research
(grant 97-05-64641) and by INTAS (grant 96-1665).
